1. Field of the Invention
This invention relates to a reference voltage generating circuit and more particularly, to a reference voltage generating circuit formed of bipolar transistors, which is used for a constant voltage circuit.
2. Description of the Prior Art
As a reference voltage generating circuit formed of bipolar transistors, conventionally, the Widlar bandgap voltage reference shown in FIG. 1 is well known and has been put to practical use. The circuit in FIG. 1 comprises a constant current source CS (constant current I) disposed on the side of a power source (supply voltage V.sup.+) and three npn-type bipolar transistors Q71, Q72 and Q73.
The transistor Q71 has an emitter directly connected to the earth, a collector connected through a first resistor (resistance R71) to the current source CS and a base connected to the collector thereof. The collector and the base of the transistor Q71 are connected in common to a base of the transistor Q72.
The transistor Q72 has an emitter connected through a third resistor (resistance R73) to the earth and a collector connected through a second resistor (resistance R72) to the current source CS.
The transistor Q73 has an emitter connected directly to the earth and a collector connected directly to the current source CS.
A reference voltage V.sub.ref is outputted from an end to which the collectors of the transistors Q71, Q72 and Q73 and the current source CS are connected in common.
The operating principle of the Widlar voltage reference shown in FIG. 1 is as follows:
If the base-to-emitter voltage of the transistor Q73 is expressed as V.sub.BE, the difference of the base-to-emitter voltages of the transistors Q71 and Q72 is expressed as .increment.V.sub.BE, the reference voltage V.sub.ref to be outputted is expressed as follows: EQU V.sub.ref =V.sub.BE +(R72/R73).multidot..increment.V.sub.BE (1)
In the other hand, if Boltzmann's constant is k, absolute temperature is T and a charge of an electron is q, the voltage difference .increment.V.sub.BE is expressed as follows: EQU .increment.V.sub.BE =(kT/q).multidot.1n(J.sub.1 /J.sub.2)
where J1 and J2 are current densities of the transistors Q71 and Q72, respectively.
Thus the reference voltage V.sub.ref is expressed as follows: EQU V.sub.ref =V.sub.BE +(R72/R73).multidot.(kT/q).multidot.1n(J.sub.1 /J.sub.2)(3)
In the Widlar bandgap voltage reference described above, the ratio of the current densities (J.sub.1 /J.sub.2) changes in response to the ambient temperature, so that the reference voltage V.sub.ref has such a temperature characteristic as shown in FIG. 2. In addition, since the circuit requires the constant current source CS and the controlling transistor Q73, the circuit is inclined to be large in scale.